Membrane fouling is generally recognized as the outstanding problem in modern membrane separations. A full discussion of the problems, specifically associated with ultrafiltration, can be found in xe2x80x9cFifteen Years of Ultrafiltrationxe2x80x9d by Micheals, A. S. in Ultrafiltration Membranes and Applications edited by A R. Cooper (American Chemical Society Symposium, Washington, 9-14 Sep. 1979, Plenum Press, New York (1980); ISBN 0-306-40548-2) where it is stated that xe2x80x9cthe problems of reduced throughput, capacity, increased power consumption, compromised separation capability, and reduced membrane service lifetime associated with macro, solute- and colloid-fouling of ultrafiltration membranes have stubbornly resisted adequate solution despite ten years of engineering experience in pilot and full-scale industrial situations.xe2x80x9d
According to Micheals, back-flushing by reverse flow of permeate in hollow-fiber membrane modules, significantly aids in unplugging of membrane pores and detachment of adhering deposits. However, there are only two specific examples of permeate back-flushing described in that text and these concern filtration of tap water and of electro-deposition paints emulsified in water.
As set forth at pages 109 to 227 of the above text, back-flushing of hollow fibers with permeate is used where operating transmembrane pressures are only about one atmosphere so that particles are not driven hard into membrane pores during the filtering process. As indicated above, permeate back-flushing has been used where the fouling species are in liquid paint emulsion droplets as these species do not wedge into the membrane pores as do solids. As the transmembrane flux is often only five to twenty liters per square meter per hour (L/m2 hr), the corresponding fluid velocity is only a few millimeters per hour and there is, therefore, no possibility of a high velocity cleaning action.
Permeate back-flushing is, in essence, a recycling process and, thus, a sacrifice of production rate is only justified when the cleaning effect is significant. Some sticky natural wastes (such as brewing residues, starch, and egg) are not removed to any appreciable extent by permeate back-flushing According to Micheals, permeate back-flushing is, by definition, a purely hydraulic flow through totally wetted pores of the ultrafiltration membrane.
In U.S. Pat. No. 4,767,539, Ford describes an improved method of back-flushing hollow fiber filters which uses a gas back-flush medium. Ford""s invention uses the back-flush gas at a pressure of about 500 kilopascals to swell the fiber from the inside and erupt around the elastic openings. This gas back-flush resulted in better removal of foulants from the surface of the membrane than did the standard permeate back-flush. The penetration of gas into the pores of a membrane is resisted by the sure tension forces of the contained wall-wetting liquid. Indeed, surface tension is conveniently measured by the breakthrough pressure needed to force a bubble out of a submerged orifice. For common systems (such as oil in hydrophobic pores or water in hydrophilic pores) the breakthrough pressure required ranges from ten kilopascals to a thousand kilopascals. The breakthrough pressures are much higher than the usual operating pressures of the filter.
In U.S. Pat. No. 5,248,424, Cote describes a frame-less array of hollow fiber membranes and a method of maintaining a clean fiber surface while filtering a substrate and withdrawing permeate. A scrubbing gas is used to sway the free floating fibers and thus minimizes or eliminates the build-up of foulants and biological organisms on the membrane surface.
Historically, difficult feed solutions with high organic and suspended solids have been treated with capillary (0.3-1.0 mm dia) or tubular element designs. While these designs are effective, they have a number of limitations, notably, the materials of construction must be chosen not just for the permeability and rejection characteristics needed to perform separations, but also for mechanical strength to withstand the feed pressures required for operation, including back-flush. The hollow fiber and tubular constructions also have relatively low packing densities of active membrane area, thus the cost per unit area is high.
All of the above-mentioned devices are manufactured in a hollow fiber configuration. While this design has many advantages, it is not as versatile and cost effective as spiral wound membrane designs. Spiral wound membrane filtration elements with microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) membranes, are used for treating drinking or process water that is relatively low in suspended solids and organic foulants.
Increasingly, more and more difficult feed solutions are being treated with spiral wound elements. Spiral wound elements have several advantages over hollow fiber and tubular elements. The membranes are cast on a woven or non-woven support substrate which lends structural strength and allows easy membrane formation. Support fabric substrates allow membranes to be formed as composites, with the base membrane providing a strong and defect free support sure over which can be coated a thin barrier layer that dictates the transport properties. Also, the spiral wound configuration has a high packing density, has a low production cost, is easily scaled up to large systems, and replacement is easy and inexpensive. In most current commercial applications using UF, NF, or RO where spiral elements can operate effectively, they have become the design of choice due to the above mentioned benefits.
However, most spiral wound elements can not be back-flushed due to failure of the adhesive seals and delimitation of the membrane film from the support fabric substrate. A recent survey of warranty literature from manufacturers of the large majority of spiral wound element filters all contain strong, unequivocal language reaffirming that any back pressure would damage the elements and are conclusive grounds for voiding the membrane element warranty. A design for spiral wound elements that would allow for effective back-flushing is needed.
It is an object of this invention to provide a spiral wound membrane and element configuration capable of being back-flushed, and an improved method of processing and back-flushing fluids using spiral wound membrane elements.
A spiral wound membrane filtration element capable of being back-flushed has a permeate carrier sheet, a membrane filter layer sheet adhesively bonded to the permeate carrier sheet, and a feed spacer sheet in between layers of membrane filter layer sheets. The membrane filter layer sheet is normally folded in half, over a feed spacer sheet. An active membrane film side of the membrane filter layer sheet faces both sides of the feed spacer. The feed spacer sheet, the membrane filter layer sheets, and the permeate carrier sheet are wrapped around a permeate collection tube.
The membrane filter layer sheet further has a support substrate with a Frazier air permeability between 1 and 10 cfm/ft2. The membrane filter layer sheet may be homogenous or asymmetric. For a homogenous membrane filter layer sheet, polymeric film may be encased around a support substrate or be a self-supporting polymeric film. In an asymmetric membrane filter layer sheet, the polymer film is cast on top of a support substrate, and adequately bonded thereto in order to eliminate delamination during a back-flush cycle.
The permeate carrier fabric sheet acts as a conduit that allows the part of the feed solution which permeates the membrane filter layer sheet to exit the element via the permeate collection tube which is in the center of the element. Each membrane filter layer sheet is bonded to the permeate carrier fabric sheet on the three sides not adjacent to the permeate collection tube with an adhesive capable of retaining the seal throughout a back-flushing of the element.
A method of making a back-flushable spiral wound membrane filtration element comprises forming a membrane filter layer sheet, cutting the membrane filter layer sheet to a desired length, placing a cut piece of a feed spacer sheet on top of the membrane filter layer sheet, the width of the feed spacer sheet being approximately half the width of the membrane filter layer sheet; folding the membrane filter layer sheet over the feed spacer so that the feed spacer sheet is sandwiched between two layers of the membrane filter layer sheet; attaching a center side part of a permeate carrier sheet to the permeate collection tube; applying an adhesive seal on the permeate carrier sheet along sides other than the center side part; positioning the membrane filter layer sheet-feed spacer sheet sandwich over the permeate carrier sheet such that the adhesive seal bonds the membrane filter layer sheet to the permeate carrier sheet; and wrapping the permeate carrier sheet, the membrane filter layer sheet, and the feed spacer sheet around the permeate collection tube.
A method of creating a permeable membrane filter layer sheet comprising placing a casting solution of a rain thickness on a passing support substrate; controlling the thickness of the casting solution on the support substrate through use of a mechanical device for dispensing the casting solution; and immersing the substrate with the casting solution into a quench bath to allow removal of casting solution after an air quench time that allows formation of a thin skin on the support substrate.
A preferred back-flush system for the back-flushable spiral wound membrane filtration element comprises a feed solution for the filtration element from a source; a pump suction pipe having a shut off valve, the pump suction pipe used in withdrawal of the feed solution; a feed pump for pumping and pressurizing the feed solution from the source through the filtration element; a feed valve for controlling the pump discharge pressure; a feed pressure gauge for measuring the feed pressure from the pump; a feed pipe through which the pressurized feed solution flows to the element; an element pressure tube wherein a first portion of the feed solution permeates the membrane filtration element as a permeate, and a second portion of the feed solution does not permeate and exits the membrane filtration element as a concentrate; a feed diverter valve for controlling flow from the feed pipe to the pressure tube; a gas tank having a gas regulator and compressed gas capable of back-flushing the membrane filtration element; a concentrate diverter valve for controlling the flow rate of concentrate out of the exiting pressure tube; a concentrate valve for controlling the flow rate of concentrate out of the concentrate diverter valve; a concentrate flow meter for measuring the concentrate flow out of the concentrate valve; a permeate accumulator capable of holding permeate for the back-flush step; a permeate diverter valve for controlling the flow rate of permeate out of the permeate accumulator and for controlling the flow rate of gas out of the tank while back-flushing; and a permeate flow meter for measuring permeate flow out of the permeate diverter valve.
Another preferred method of back-flushing the spiral wound membrane filtration element through the back-flush system comprises gathering feed solution for the filtration element from a source; withdrawing feed solution from the source through a pump suction pipe having a shut off valve; pumping the feed solution from the source using a feed pump; pressurizing the feed solution in the pump; pumping the pressed feed solution through the filtration element using the feed pump; controlling the pump discharge pressure using a feed valve; measuring the feed pressure from the pump using a feed pressure gauge; controlling flow from a feed pipe to the element using a feed diverter valve; permeating the membrane filtration element with a portion of the feed solution as a permeate; allowing a second portion of the feed solution which does not permeate the membrane filtration element to pass through the membrane filtration element as a concentrate; pressurizing back flush fluid in a gas tank used for back-flushing the membrane filtration element; controlling the flow rate of concentrate out of the element using a concentrate diverter valve; controlling the flow rate of concentrate out of the concentrate diverter valve using a concentrate valve; measuring the concentrate flow out of the concentrate valve using a concentrate flow meter; holding permeate for the back-flush step using a permeate accumulator; controlling the flow rate of permeate out of the permeate accumulator and the flow rate of gas out of the tank while back-flushing using a permeate diverter valve; measuring permeate flow out of the permeate diverter valve using a permeate flow meter; and cleaning the spiral wound membrane filtration element having a membrane with pores through back-flushing the element.
Methods of cleaning a spiral wound membrane filtration element and system useful for filtering feed solution are carried out by pressurizing the feed solution or by creating a vacuum in the permeate collection tube, and periodically introducing a pressurized back flush fluid into the permeate collection tube of the filtration element to back-flush the membrane, under a pressure and for a time sufficient to dislodge a substantial portion of the retained solids on the surface of the membrane.